1. Field of the Invention
The present invention relates to a laminated porous film and more particularly to a laminated porous film which can be utilized as packing, hygiene, livestock, agricultural, building, medical products, separation films, light diffusing plates, separators for batteries and preferably utilized as separators for nonaqueous electrolyte secondary batteries.
2. Description of the Related Art
A secondary battery is widely used as the power source of OA, FA, household appliances, and portable devices such as communication instruments. The secondary battery has a favorable volumetric efficiency when it is mounted on devices and allows the devices to be compact and lightweight. Therefore there is an increase in the use of portable devices in which a lithium-ion rechargeable battery is mounted. Owing to research and development of a large secondary battery which have been made in many fields relating to the problem of energy and environment such as load leveling, UPS, an electric car, and the like, the secondary battery is allowed to have a large capacity, a high output, a high voltage, and an excellent long-term storage stability. Therefore the lithium-ion rechargeable battery which is a kind of the nonaqueous electrolyte secondary battery has widely spread in its use.
The upper limit of the working voltage of the lithium-ion rechargeable battery is usually set to 4.1V to 4.2V. Because electrolysis occurs in an aqueous solution at such a high voltage, the aqueous solution cannot be used as an electrolyte. Therefore as an electrolyte capable of withstanding a high voltage, a so-called nonaqueous electrolyte in which an organic solvent is used is adopted.
As a solvent for the nonaqueous electrolyte, an organic solvent having a high permittivity which allows a large number of lithium ions to be present is widely used. Organic carbonate ester such as polypropylene carbonate or ethylene carbonate is mainly used as the organic solvent having a high permittivity. As a supporting electrolyte serving as the ion source of the lithium ion in the solvent, an electrolyte having a high reactivity such as lithium phosphate hexafluoride is used in the solvent by melting it.
A separator is interposed between the positive electrode of the lithium-ion rechargeable battery and its negative electrode to prevent an internal short circuit from occurring. Needless to say, the separator is demanded to have insulating performance as its role. In addition the separator is required to have a porous structure so that it has air permeability to allow the movement of the lithium ion and a function of diffusing and holding the electrolyte. To satisfy these demands, a porous film is used as the separator.
Because batteries having a high capacity are used recently, the degree of importance for the safety of the battery has increased.
A shut-down characteristic (hereinafter referred to as SD characteristic) contributes to the safety of the separator for the battery. The SD characteristic has a function of closing fine pores when the battery has a high temperature of 100° C. to 150° C. As a result, the ion conduction inside the battery is cut off. Thereby it is possible to prevent the temperature inside the battery from rising. To use the porous film as the separator for the battery, it is necessary for the battery to have the SD characteristic.
As other characteristic contributing to the safety of the separator for the battery, a break-down characteristic (hereinafter referred to as BD characteristic) is used. The BD characteristic has a function of preventing the film from being broken and keeping separating the positive pole and the negative pole from each other even when the battery becomes abnormal in its operation and has thermo runaway that the temperature of the battery becomes not less than 200° C. The BD characteristic allows insulation to be maintained even at a high temperature and prevents a wide range of short circuit from occurring between the electrodes, thereby preventing the occurrence of an accident such as firing caused by an abnormal heat generation of the battery. Therefore to use the porous film as the separator for the battery, it is preferable for the battery to have the BD characteristic. It is also preferable that the break-down characteristic indicating a lowest temperature at which the separator is broken is as high as possible.
In the method of producing the separator for the battery disclosed in U.S. Pat. No. 2,883,726 (patent document 1), the film consisting of polyethylene and polypropylene layered one upon another is made porous by stretching the film in one axial direction at two stages by changing temperature.
The separator produced by the above-described method contains the polypropylene layer whose crystal fusion peak temperature is higher than that of a conventional separator consisting of a monolayer of the polyethylene. Thus the separator is heat-resistant and is thus advantageous because the separator has a high BD characteristic. But with a recent high energy density of the battery, the polypropylene is insufficient in its heat resistance. The separator is demanded to display the BD characteristic at high temperatures.
In addition the separator produced by the above-described method has a problem in its strength. Specifically the separator is weak when it is torn in the same direction as a stretching direction. That is, the separator is liable to crack in the stretching direction.
The above-described production method has also a problem that a strict control is required for the production conditions and thus the productivity is unpreferable.
In Examined Japanese Patent Application Nos. 6-84450 (patent document 2) and U.S. Pat. No. 2,509,030 (patent document 3), to enhance the transparency of the porous film, there are proposed methods for obtaining the porous film by stretching the propylene sheet containing much β crystal which is one of the crystalline morphology.
But the porous film consisting of the polypropylene is insufficient in its heat resistance. Thus there is a demand for the development of a film capable of displaying the BD characteristic at a higher temperature.    Patent document 1: U.S. Pat. No. 2,883,726    Patent document 2: Examined Japanese Patent Application Laid-Open No. 6-84450    Patent document 3: U.S. Pat. No. 2,509,030